The overall objective of Project 4 is to improve survival after hematopoietic stem cell transplantation by eliminating mortality due to the liver, renal, and pulmonary toxicity of myeloablative preparative regimens that contain cyclophosphamide (CY). In previous studies we found that exposure to high levels of carboxyethyl phosphoramide mustard, a metabolite of CY, is associated with a striking increase in transplant-related mortality. Thus, our strategy to reduce toxicity is to normalize patient exposure to toxic metabolites of CY. Our first aim is to develop a method to ensure that patients receiving CY and total body irradiation as preparative therapy for hematological malignancies are dosed with CY according to how it is metabolized, using CY kinetics from day 1 to adjust the CY dose on day 2. When this method is realized, we will conduct a phase III trial of targeted CY dosing, with overall survival as the primary endpoint. Our second aim is to relate the observed CY metabolism phenotype (that is, abnormally high exposure to carboxyethyl phosphoramide mustard) to its genetic determinants. We will probe genes from the DNA from patients at the highest and lowest quartiles of CY metabolite exposure for allelic variations relevant to six metabolizing enzymes and hepatocyte transporters of CY metabolites. The genes coding for enzymes or transporters most likely to control exposure to toxic metabolites of CY will be explored with highest priority, that is, variations in the hepatocyte anion transporters ABCC2 (MRP-2) and ABCC1 (MRP-1), which export CY conjugates from hepatocytes into bile and blood, respectively. Our third aim is to determine whether aberrant metabolism of CY is related to regimenrelated toxicity in patients receiving conditioning with busulfan and CY, similar to the relationship already shown for CY/TBI patients. In summary, we have had success in limiting the toxicity of busulfan by targeting busulfan doses to steady state concentrations, and believe that we can achieve a similar result by targeting the dose of cyclophosphamide. It now seems possible to essentially eliminate organ failure as a cause of non-relapse mortality after cyclophosphamide-containing myeloablative regimens.